Human deafness results from numerous factors including trauma, ear infections, congenital factors, toxic effects of some antibiotics, and from diseases such as meningitis. Sensorineural damage (damage to the hair cells in the cochlea) is the largest single form of hearing loss. In a healthy ear these hair cells convert acoustic signals in the inner ear to electrical signals that can be interpreted by the brain as sound. It is estimated that over 7% of the U.S. population is affected by sensorineural deafness, and one in a thousand infants is born totally deaf. Extrapolating these percentage figures, it is estimated that there are 30 million people in the world who are profoundly deaf.
Considerable research over the past several decades has been directed towards developing a means to bypass the non-functioning hair cells in the inner ear (or cochlea) by using electrodes to directly stimulate auditory afferent neurons within the cochlea. This so called cochlear implant technology has progressed from early methods of attaching one or more single wire electrodes onto the promontory or the bony shell of the cochlea, to drilling directly into the cochlea, and inserting electrodes into the scalae therein. Electrodes used in modern cochlear prostheses generally use a longitudinal monopolar (or bipolar) electrode configuration where small platinum/iridium plates or circular platinum rings connected internally by thin wires, with the electrodes and wires held together in a smooth elongated silicone carrier, are surgically implanted into the scala tympani (one of the canals within the cochlea), via a hole made in the mastoid bone behind the ear. Entry into the scala tympani is generally via the round window membrane. The electrodes are electrically connected to an electronics package anchored in a cavity made in the mastoid bone. Information is sent to this internal electronics package transcutaneously, via RF transmission across the skin barrier, from an external body-worn (generally behind the ear) electronics package that houses the speech processor, control electronics and power supply.
Current cochlear implant systems include an implant portion and an external portion. The implant portion typically includes: (1) an electrode array, (2) an implanted coil and (3) a hermetically-sealed housing to which the electrode array and implanted coil are attached and in which electronic circuitry, e.g., data processing circuitry and pulse generator circuitry are housed. The external portion typically includes: (1) a microphone, (2) a battery-powered sound processor for processing the signals sensed by the microphone and for generating control and other signals that are transmitted to the implant portion and (3) a headpiece, connected to the sound processor by way of a cable or wire(s), in which an external coil is housed. In operation, the headpiece coil (external coil) is inductively coupled with the implanted coil so that power and data can be transferred to the implant portion from the external portion.
U.S. Pat. No. 5,123,422 teaches the use of internal hinges or slits, where such hinges or slits are oriented to give flexibility in only one plane, and can be inserted in the scala tympani without curling, thus orienting the electrode sites “to obtain good stimulation of the nerve cells”. U.S. Pat. No. 4,261,372 uses “V” shaped notches along one side of the array to permit the array to assume the required curved shape within the scala, and to obtain greater insertion depth of the electrodes by first inserting one part of the electrode into the first turn of the scala tympani and then inserting the other part into the second turn of the scala tympani. U.S. Pat. No. 4,832,051 describes an electrode device where “the elements are resiliently attached together so that the stack of elements is stiff in compression along the common axis and is flexible in tension.” Cochlear stimulation devises have been further described in U.S. Pat. Nos. 7,194,314, 7,085,605, 6,906,262, 6,782,619, 6,678,564, or 6,374,143.
The electrode is an important part of cochlear implant system because it affects the current spread and the response of the auditory nerves. Modern technology uses multi-channel (electrode) implants as opposed to single electrode implants, as the former provides electrical stimulation at multiple sites in the cochlea using an array of electrodes. An electrode array is used so that different auditory nerve fibers can be stimulated at different places in the cochlea, thereby exploiting the place mechanism for coding frequencies. Different electrodes are stimulated depending on the frequency of the signal. Electrodes near the base of the cochlea are stimulated with high frequency signals, while electrodes near the apex are stimulated with low frequency signals. The design of the electrode array are important with regard to the electrode placement, number of electrodes and spacing between electrodes, orientation of electrodes with respect to the excitable tissue and electrode configuration. The electrodes are commonly placed in the scala tympani because it brings the electrodes in close proximity with auditory neurons which lie along the length of the cochlea and thereby preserves the place mechanism for coding frequencies. The larger the number of electrodes, the finer the place resolution for coding frequencies. However, using a large number of electrodes will not necessarily result in better performance, because frequency coding is constrained by the number of surviving auditory neurons that can be stimulated. Studies have shown that for adequate speech perception, at least 8 electrodes are required.
Commercially available cochlear implant devices comprise simple tapered longitudinal bipolar and monopolar electrode arrays using small platinum/iridium balls or circular rings. However these devices were developed taking into consideration the ease of fabrication as well as surgical insertibility rather than the critical design parameters necessary to achieve optimum neural stimulation. They continue to be hand built under a microscope and are made using wire based technologies. There is, therefore, still a need in the art to improve the currently available cochlear stimulation devices by appropriate electrode design consideration.
The present invention is an improvement over the known commercial devices. The device comprises an electrode array designed to increase the current transfer capability of the electrodes by using high surface area electrodes without increasing the geometrical surface area. Since an implantable stimulation or sensing electrode is intended for long term use in a neural stimulator with a low power consumption and limited compliance voltages, it requires high electrode capacitance and correspondingly low electrical impedance. Without sufficiently low impedance, a large voltage may cause polarization of both the electrode and the tissue to which the electrode is attached forming possible harmful byproducts, degrading the electrode and damaging the tissue.